The concept that the cancer patient surmounts some defensive reaction against his tumor is well ingrained in medical thinking. The most dramatic form of this defense is a spontaneous regression of cancer. However, spontaneous regression is a very rare phenomenon. Recent years have seen an enormous growth in our understanding of the role of antibodies and delayed hypersensitivity in human disease. The monocyte and macrophage represent another facet in the matrix of the host response to foreign material. It has been well established that monocytes are capable of detecting small amounts of gamma globulin on the surface of a red cell and that they adhere to the red cell surface and alter it structurally. Macrophages may also recognize an abnormality distinct to the surface of cancer cells. This interaction between macrophages and cancer cells may be important in the host defense against cancer. By understanding this process we may find the means to stimulate it and thereby aid the patient with cancer in his fight against his disease. Therefore, we propose to study the interaction between monocytes-macrophages and cancer cells by focusing first on the red cell model and advancing to the cancer cell, drawing our attention initially to the well-defined monocyte receptor for gamma globulin and advancing to the less well-defined macrophage receptor for the cancer cell surface. During the conduct of these studies, we hope to define the nature of immune membrane damage in cancer cells and red cells and to characterize the macrophage membrane receptor which mediates this damage.